Generally, an image sensor is a semiconductor device that converts an optical image into an electrical signal. The image sensor may be classified as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) image sensor.
The CMOS image sensor is fabricated using CMOS technology, and thus has low power consumption. Also, the CMOS image sensor is capable of being highly integrated and can be fabricated as a slim sized product.
Therefore, the CMOS image sensor has been widely used for various applications such as a digital still camera and a digital video camera.
Hereinafter, a related art CMOS image sensor will be described with reference to FIG. 1.
FIG. 1 is a plan view of a related art CMOS image sensor including four transistors.
The CMOS image sensor has a unit pixel incorporating a photodiode 12 for sensing light and four N-type MOS (NMOS) transistors.
The photodiode 12 is formed at one end of an active region 10. The four transistors constitute a transfer transistor Tx, a reset transistor Rx, a drive transistor Dx, and a select transistor Sx.
In FIG. 1, reference numerals 14, 16, 18 and 20 refer to gate patterns for the transfer transistor, reset transistor, drive transistor, and select transistor, respectively.
In operation, the photodiode 12 absorbs light to generate electrons.
When the transfer transistor Tx is turned on, the generated electrons move to a floating diffusion (FD) region 22.
The electrons moving to the FD region 22 generate an output signal through a source follower transistor (the drive transistor Dx).
In the related art CMOS image sensor, the FD region 22 is connected with the drive transistor Dx using a metal wiring 24.
Therefore, a metal contact hole 26 should be formed in the FD region 22 and on the gate electrode 18 of the drive transistor Dx.
However, the silicon substrate receives damage during an etch process for forming the metal contact hole 26.
In addition, in the related art CMOS image sensor, the FD region 22 is salicided prior to the forming of the metal wiring 24.
However, when the FD region 22 is salicided, leakage current may increase, and thus the electrons flowing in the FD region 22 may be lost or a dark current may be generated.
Accordingly, in the related art CMOS image sensor, the FD region 22 is connected with the drive transistor Dx through a non-salicide contact (i.e. not using a salicide contact).
However, such a non-salicide contact leads to an increase of contact resistance.
Also, since the non-salicided contact region has a higher etch rate than the salicide region, the silicon substrate may be etched during an etch process which may cause a leakage current.